Exhaust system for a lean-burn IC engine comprising a PGM component and a SCR catalyst

ABSTRACT

An exhaust system  10  for a vehicular lean-burn internal combustion engine comprises:
         (a) a first substrate monolith  6  comprising a SCR catalyst;   (b) at least one second substrate monolith  4  comprising a catalytic washcoat coating comprising at least one platinum group metal (PGM) disposed upstream of the first substrate monolith; and   (c) a third substrate monolith  2  disposed between the first substrate monolith and the or each second substrate monolith,
 
wherein at least one PGM on the or each second substrate monolith  4  is liable to volatilise when the or each second substrate monolith  4  is exposed to relatively extreme conditions including relatively high temperatures, and wherein the third substrate monolith  2  comprises a washcoat coating comprising at least one metal oxide for trapping volatilised PGM.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit to U.S. Provisional PatentApplication No. 61/569,530 filed on Dec. 12, 2011, and Great BritainPatent Application No. 1200781.1 filed on Jan. 18, 2012, both of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an exhaust system for a vehicularlean-burn internal combustion engine, which system comprising a firstsubstrate monolith comprising a SCR catalyst and at least one secondsubstrate monolith comprising at least one platinum group metal (PGM)disposed upstream of the first substrate monolith.

BACKGROUND TO THE INVENTION

Generally, there are four classes of pollutant that are legislatedagainst by inter-governmental organisations throughout the world: carbonmonoxide (CO), unburned hydrocarbons (HC), oxides of nitrogen (NO_(x))and particulate matter (PM).

As emissions standards for permissible emission of such pollutants inexhaust gases from vehicular engines become progressively tightened, acombination of engine management and multiple catalyst exhaust gasaftertreatment systems are being proposed and developed to meet theseemission standards. For exhaust systems containing a particulate filter,it is common for engine management to be used periodically (e.g. every500 km) to increase the temperature in the filter in order to combustsubstantially all remaining soot held on the filter thereby to returnthe system to a base-line level. These engine managed soot combustionevents are often called “filter regeneration”. While a primary focus offilter regeneration is to combust soot held on the filter, an unintendedconsequence is that one or more catalyst coatings present in the exhaustsystem, e.g. a filter coating on the filter itself (a so-calledcatalysed soot filter (CSF)) an oxidation catalyst (such as a dieseloxidation catalyst (DOC)) or a NO_(x) adsorber catalyst (NAC) locatedupstream or downstream of the filter (e.g. a first DOC followed by adiesel particulate filter, followed in turn by a second DOC and finallya SCR catalyst) can be regularly exposed to high exhaust gastemperatures, depending on the level of engine management control in thesystem. Such conditions may also be experienced with unintendedoccasional engine upset modes or uncontrolled or poorly controlledregeneration events. However, some diesel engines, particularly heavyduty diesel engines operating at high load, may even expose catalysts tosignificant temperatures, e.g. >600° C. under normal operatingconditions.

As vehicle manufacturers develop their engines and engine managementsystems for meeting the emission standards, the Applicant/Assignee isbeing asked by the vehicle manufacturers to propose catalytic componentsand combinations of catalytic components to assist in the goal ofmeeting the emission standards. Such components include DOCs foroxidising CO, HCs and optionally NO also; CSFs for oxidising CO, HCs,optionally for oxidising NO also, and for trapping particulate matterfor subsequent combustion; NACs for oxidising CO and HC and foroxidising nitrogen monoxide (NO) and absorbing it from a lean exhaustgas and to desorb adsorbed NO_(x) and for reducing it to N₂ in a richexhaust gas (see below); and selective catalytic reduction (SCR)catalysts for reducing NO_(x) to N₂ in the presence of a nitrogenousreductant, such as ammonia (see below).

In practice, catalyst compositions employed in DOCs and CSFs are quitesimilar. Generally, however, a principle difference between the use of aDOC and a CSF is the substrate monolith onto which the catalystcomposition is coated: in the case of a DOC, the substrate monolith istypically a flow-through substrate monolith, comprising a metal orceramic honeycomb monolith having an array of elongate channelsextending therethrough, which channels are open at both ends; a CSFsubstrate monolith is a filtering monolith such as a wall-flow filter,e.g. a ceramic porous filter substrate comprising a plurality of inletchannels arranged in parallel with a plurality of outlet channels,wherein each inlet channel and each outlet channel is defined in part bya ceramic wall of porous structure, wherein each inlet channel isalternately separated from an outlet channel by a ceramic wall of porousstructure and vice versa. In other words, the wall-flow filter is ahoneycomb arrangement defining a plurality of first channels plugged atan upstream end and a plurality of second channels not plugged at theupstream end but plugged at a downstream end. Channels vertically andlaterally adjacent to a first channel are plugged at a downstream end.When viewed from either end, the alternately plugged and open ends ofthe channels take on the appearance of a chessboard.

Quite complicated multiple layered catalyst arrangements such as DOCsand NACs can be coated on a flow-through substrate monolith. Although itis possible to coat a surface of a filter monolith, e.g. an inletchannel surface of a wall-flow filter, with more than one layer ofcatalyst composition, an issue with coating filtering monoliths is toavoid unnecessarily increasing back-pressure, when in use, byoverloading the filter monolith with catalyst washcoat, therebyrestricting the passage of gas therethrough. Hence, although coating asurface of a filter substrate monolith sequentially with one or moredifferent catalyst layers is not impossible, it is more common fordifferent catalyst compositions to be segregated either in zones, e.g.axially segregated front and rear half zones of a filter monolith, orelse by coating an inlet channel of a wall-flow filter substratemonolith with a first catalyst composition and an outlet channel thereofwith a second catalyst composition. However, in particular embodimentsof the present invention, the filter inlet is coated with one or morelayers, which layers may be the same or a different catalystcomposition. It has also been proposed to coat a NAC composition on afiltering substrate monolith (see e.g. EP 0766993).

In exhaust systems comprising multiple catalyst components, eachcomprising a separate substrate monolith, typically, the SCR catalyst islocated downstream of a DOC and/or a CSF and/or a NAC because it isknown that by oxidising some nitrogen oxide (NO) in the exhaust gas tonitrogen dioxide (NO₂) so that there is about a 1:1 ratio of NO:NO₂exiting the DOC and/or the CSF and/or the NAC, the downstream SCRreaction is promoted (see below). It is also well known from EP341832(the so-called Continuously Regenerating Trap or CRT®) that NO₂,generated by oxidising NO in exhaust gas to NO₂, can be used to combustsoot passively on a downstream filter. In exhaust system arrangementswhere the process of EP341832 is important, were the SCR catalyst to belocated upstream of the filter, this would reduce or prevent the processof combusting trapped soot in NO₂, because a majority of the NO_(x) usedfor combusting the soot would likely be removed on the SCR catalyst.

However, a preferred system arrangement for light-duty diesel vehiclesis a diesel oxidation catalyst (DOC) followed by a nitrogenous reductantinjector, then a SCR catalyst and finally a catalysed soot filter (CSF).A short hand for such an arrangement is “DOC/SCR/CSF”. This arrangementis preferred for light-duty diesel vehicles because an importantconsideration is to achieve NO_(x) conversion in an exhaust system asquickly as is possible after a vehicle engine is started to enable (i)precursors of nitrogenous reductants such as ammonia to beinjected/decomposed in order to liberate ammonia for NO_(x) conversion;and (ii) as high NO_(x) conversion as possible. Were a large thermalmass filter to be placed upstream of the SCR catalyst, i.e. between theDOC and the SCR catalyst (“DOC/CSF/SCR”), (i) and (ii) would take farlonger to achieve and NO_(x) conversion as a whole of the emissionstandard drive cycle could be reduced. Particulate removal can be doneusing oxygen and occasional forced regeneration of the filter usingengine management techniques.

It has also been proposed to coat a SCR catalyst washcoat on a filtersubstrate monolith itself (see e.g. WO 2005/016497), in which case anoxidation catalyst may be located upstream of the SCR-coated filtersubstrate (whether the oxidation catalyst is a component of a DOC, a CSFor a NAC) in order to modify the NO/NO₂ ratio for promoting NO_(x)reduction activity on the SCR catalyst. There have also been proposalsto locate a NAC upstream of a SCR catalyst disposed on a flow-throughsubstrate monolith, which NAC can generate NH₃ in situ duringregeneration of the NAC (see below). One such proposal is disclosed inGB 2375059.

NACs are known e.g. from U.S. Pat. No. 5,473,887 and are designed toadsorb NO_(x) from lean exhaust gas (lambda>1) and to desorb the NO_(x)when the oxygen concentration in the exhaust gas is decreased. DesorbedNO_(x) may be reduced to N₂ with a suitable reductant, e.g. engine fuel,promoted by a catalyst component, such as rhodium, of the NAC itself orlocated downstream of the NAC. In practice, control of oxygenconcentration can be adjusted to a desired redox compositionintermittently in response to a calculated remaining NO_(x) adsorptioncapacity of the NAC, e.g. richer than normal engine running operation(but still lean of stoichiometric or lambda=1 composition),stoichiometric or rich of stoichiometric (lambda<1). The oxygenconcentration can be adjusted by a number of means, e.g. throttling,injection of additional hydrocarbon fuel into an engine cylinder such asduring the exhaust stroke or injecting hydrocarbon fuel directly intoexhaust gas downstream of an engine manifold.

A typical NAC formulation includes a catalytic oxidation component, suchas platinum, a significant quantity, (i.e. substantially more than isrequired for use as a promoter such as a promoter in a three-waycatalyst), of a NO_(x)-storage component, such as barium, and areduction catalyst, e.g. rhodium. One mechanism commonly given forNO_(x)-storage from a lean exhaust gas for this formulation is:NO+1/2O₂→NO₂  (1); andBaO+2NO₂+1/2O₂→Ba(NO₃)₂  (2),wherein in reaction (1), the nitric oxide reacts with oxygen on activeoxidation sites on the platinum to form NO₂. Reaction (2) involvesadsorption of the NO₂ by the storage material in the form of aninorganic nitrate.

At lower oxygen concentrations and/or at elevated temperatures, thenitrate species become thermodynamically unstable and decompose,producing NO or NO₂ according to reaction (3) below. In the presence ofa suitable reductant, these nitrogen oxides are subsequently reduced bycarbon monoxide, hydrogen and hydrocarbons to N₂, which can take placeover the reduction catalyst (see reaction (4)).Ba(NO₃)₂→BaO+2NO+3/2O₂ or Ba(NO₃)₂→BaO+2NO₂+1/2O₂  (3); andNO+CO→1/2N₂+CO₂  (4);

(Other reactions include Ba(NO₃)₂+8H₂→BaO+2NH₃+5H₂O followed byNH₃+NO_(x)→N₂+yH₂O or 2NH₃+2O₂+CO→N₂+3H₂O+CO₂ etc.).

In the reactions of (1)-(4) inclusive herein above, the reactive bariumspecies is given as the oxide. However, it is understood that in thepresence of air most of the barium is in the form of the carbonate orpossibly the hydroxide. The skilled person can adapt the above reactionschemes accordingly for species of barium other than the oxide andsequence of catalytic coatings in the exhaust stream.

Oxidation catalysts promote the oxidation of CO to CO₂ and unburned HCsto CO₂ and H₂O. Typical oxidation catalysts include platinum and/orpalladium on a high surface area support.

The application of SCR technology to treat NO_(x) emissions fromvehicular internal combustion (IC) engines, particularly lean-burn ICengines, is well known. Examples of nitrogenous reductants that may beused in the SCR reaction include compounds such as nitrogen hydrides,e.g. ammonia (NH₃) or hydrazine, or an NH₃ precursor.

NH₃ precursors are one or more compounds from which NH₃ can be derived,e.g. by hydrolysis. Decomposition of the precursor to ammonia and otherby-products can be by hydrothermal or catalytic hydrolysis. NH₃precursors include urea (CO(NH₂)₂) as an aqueous solution or as a solidor ammonium carbamate (NH₂COONH₄). If the urea is used as an aqueoussolution, a eutectic mixture, e.g. a 32.5% NH₃ (aq), is preferred.Additives can be included in the aqueous solutions to reduce thecrystallisation temperature. Presently, urea is the preferred source ofNH₃ for mobile applications because it is less toxic than NH₃, it iseasy to transport and handle, is inexpensive and commonly available.Incomplete hydrolysis of urea can lead to increased PM emissions ontests for meeting the relevant emission test cycle because partiallyhydrolysed urea solids or droplets will be trapped by the filter paperused in the legislative test for PM and counted as PM mass. Furthermore,the release of certain products of incomplete urea hydrolysis, such ascyanuric acid, is environmentally undesirable.

SCR has three main reactions (represented below in reactions (5)-(7)inclusive) which reduce NO_(x) to elemental nitrogen.4NH₃+4NO+O₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO)  (5)4NH₃+2NO+2NO₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO_(x))  (6)8NH₃+6NO₂→7N₂+12H₂O (i.e. 4:3 NH₃:NO_(x))  (7)

A relevant undesirable, non-selective side-reaction is:2NH₃+2NO₂→N₂O+3H₂O+N₂  (8)

In practice, reaction (7) is relatively slow compared with reaction (5)and reaction (6) is quickest of all. For this reason, when skilledtechnologists design exhaust aftertreatment systems for vehicles, theyoften prefer to dispose an oxidation catalyst element (e.g. a DOC and/ora CSF and/or a NAC) upstream of an SCR catalyst.

when certain DOCs and/or NACs and/or CSFs become exposed to the hightemperatures encountered during filter regeneration and/or an engineupset event and/or (in certain heavy-duty diesel applications) normalhigh temperature exhaust gas, it is possible given sufficient time athigh temperature for low levels of platinum group metal components,particularly Pt, to volatilise from the DOC and/or the NAC and/or theCSF components and subsequently for the platinum group metal to becometrapped on a downstream SCR catalyst. This can have a highly detrimentaleffect on the performance of the SCR catalyst, since the presence of Ptleads to a high activity for competing, non-selective ammonia oxidationsuch as in reaction (9) (which shows the complete oxidation of NH₃),thereby producing secondary emissions and/or unproductively consumingNH₃.4NH₃+5O₂→4NO+6H₂O  (9)

One vehicle manufacturer has reported the observation of this phenomenonin SAE paper 2009-01-0627, which is entitled “Impact and Prevention ofUltra-Low Contamination of Platinum Group Metals on SCR catalysts Due toDOC Design” and includes data comparing the NO_(x) conversion activityagainst temperature for a Fe/zeolite SCR catalyst located in seriesbehind four suppliers' platinum group metal (PGM)-containing DOCs thatwere contacted with a flowing model exhaust gas at 850° C. for 16 hours.The results presented show that the NO_(x) conversion activity of aFe/zeolite SCR catalyst disposed behind a 20Pt:Pd DOC at 70 gft⁻³ totalPGM was negatively altered at higher evaluation temperatures as comparedto lower evaluation temperatures as a result of Pt contamination. Two2Pt:Pd DOCs from different suppliers at 105 gft⁻³ total PGM were alsotested. In a first 2Pt:Pd DOC, the SCR catalyst activity was affected toa similar extent as the test on the 20Pt:Pd DOC, whereas for the second2Pt:Pd DOC tested the SCR catalyst activity was contaminated to a lesserextent, although the second 2Pt:Pd DOC still showed reduced NO_(x)conversion activity compared with the blank control (no DOC, just a baresubstrate). The authors concluded that the supplier of the second 2Pt:PdDOC, which showed more moderate NO_(x) conversion degradation, was moresuccessful in stabilising the 70 gft⁻³ Pt present with the 35 gft⁻³ Pd.A Pd-only DOC at 150 gft⁻³ demonstrated no impact on the downstream SCRrelative to the blank control. Earlier work from the authors of SAE2009-01-0627 was published in SAE paper no. 2008-01-2488.

SUMMARY OF THE INVENTION

Vehicle manufacturers have begun asking the Applicant for measures tosolve the problem of volatilisation of relatively low levels PGMs fromcomponents upstream of SCR catalysts. It would be highly desirable todevelop strategies to prevent this PGM movement onto a downstream SCRcatalyst at high temperatures. The present inventors have developed anumber of strategies for meeting this need.

The inventors have found that volatilisation of platinum from aPGM-containing catalyst comprising both platinum and palladium can occurunder extreme temperature conditions when the weight ratio of Pt:Pd isgreater than about 2:1. It is also believed that where the PGM consistsof platinum, platinum volatilisation may also be observed. The presentinventors have devised an exhaust system arrangement for use incombination with a downstream SCR catalyst which avoids or reduces theproblem of PGM, particularly Pt, migrating from an upstream relativelyhighly loaded Pt catalyst to a downstream SCR catalyst.

According to a first aspect, the invention provides an exhaust systemfor a vehicular lean-burn internal combustion engine, which systemcomprising:

-   (a) a first substrate monolith comprising a SCR catalyst;-   (b) at least one second substrate monolith comprising a catalytic    washcoat coating comprising at least one platinum group metal (PGM),    which at least one second substrate monolith is disposed upstream of    the first substrate monolith; and-   (c) a third substrate monolith disposed between the first substrate    monolith and the or each second substrate monolith,    wherein at least one PGM on the or each second substrate monolith is    liable to volatilise when the or each second substrate monolith is    exposed to relatively extreme conditions including relatively high    temperatures, and wherein the third substrate monolith comprises a    washcoat coating comprising at least one material for trapping    volatilised PGM. In general, it is preferred that the at least one    material for trapping volatilised PGM is a metal oxide.

According to a further aspect, there is provided a lean-burn internalcombustion engine, preferably a compression ignition engine, comprisingan exhaust system according to the invention.

According to a further aspect, there is provided a vehicle comprising anengine according to the invention.

According to a further aspect, the invention provides method of reducingor preventing a selective catalytic reduction (SCR) catalyst disposed ona first substrate monolith in an exhaust system of a lean-burn internalcombustion engine from becoming poisoned with platinum group metal (PGM)which may volatilise from a catalyst composition comprising PGM disposedon at least one second substrate monolith upstream of the SCR catalystwhen the catalyst composition comprising PGM is exposed to relativelyextreme conditions including relatively high temperatures, which methodcomprising adsorbing volatilised PGM in at least one PGM trappingmaterial, which is disposed on a third substrate monolith.

A further aspect of the invention relates to the use of a substratemonolith (e.g. a third substrate monolith) to reduce or preventpoisoning of a selective catalytic reduction (SCR) catalyst by aplatinum group metal (PGM), typically in an exhaust system of alean-burn internal combustion engine, wherein the substrate monolithcomprises at least one material for trapping volatilised PGM, andwherein the substrate monolith is disposed between a first substratemonolith and at least one second substrate monolith, wherein the firstsubstrate monolith comprises a SCR catalyst, and the at least one secondsubstrate monolith comprises a catalytic washcoat coating comprising atleast one platinum group metal (PGM). Typically, the at least one secondsubstrate monolith is disposed upstream of the first substrate monolith.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, reference ismade to the following Examples by way of illustration only and withreference to the accompanying drawings.

FIG. 1 is a schematic drawing of a laboratory reactor used for testingplatinum contamination on an Fe/Beta zeolite or a Cu/CHA SCR catalyst.

FIG. 2 is a graph comparing the NO_(x) conversion activity as a functionof temperature of five aged SCR catalyst cores each of which has beenaged in a laboratory-scale exhaust system configuration containing acore of Comparative Example 2 downstream of which was a Guard Bed corefrom each of Examples 4A, 4B, 5A and 5B. The results of the aged SCRactivity are plotted against activity of a fresh, i.e. un-aged Fe/BetaSCR catalyst and a control of Comparative Example 2 (no Guard Bed).

FIG. 3 is a graph comparing the NO_(x) conversion activity as a functionof temperature of a further three aged SCR catalyst cores each of whichhas been aged in a laboratory-scale exhaust system configurationcontaining a core of Comparative Example 2 downstream of which was aGuard Bed core from each of Examples 3A, 3B and 6. The results of theaged SCR activity are plotted against activity of a fresh, i.e. un-agedFe/Beta SCR catalyst and a control of Comparative Example 2 (no GuardBed).

FIG. 4 is a graph plotting the results of NO_(x) conversion activity asa function of temperature for a fresh Fe/Beta zeolite SCR catalystcompared with the activity of Fe/Beta zeolite SCR catalysts aged in thelaboratory scale exhaust system shown in FIG. 1 containing catalysedsoot filter cores of Comparative Example 2 and Examples 7 and 8.

FIG. 5 is a bar chart comparing the NO_(x) conversion activity as afunction of temperature of two aged Cu/CHA SCR catalyst cores each ofwhich has been aged in the laboratory-scale exhaust system shown in FIG.1 containing core samples of the diesel oxidation catalyst ofComparative Example 8 heated in a tube furnace at 900° C. for 2 hours ina flowing synthetic exhaust gas with the Cu/CHA zeolite SCR catalystcore held at 300° C. located downstream.

FIGS. 6A and 6B are schematic drawings of exhaust system embodimentsaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In general, the catalytic washcoat coating comprising at least one PGMcomprises one or more support materials for the PGM. The catalyst istypically applied to the or each second substrate monolith as a washcoatslurry comprising at least one PGM salt and one or more support materialfor supporting the at least one PGM in the finished catalyst coating,before the or each coated substrate monolith is dried and then calcined.The one or more material for supporting the at least one PGM may bereferred to as a “washcoat component”. It is also possible for at leastone PGM to be pre-fixed to one or more support material prior to itbeing slurried in an aqueous medium prior to coating, or for acombination of support material particles pre-fixed with PGM to beslurried in a solution of PGM salt.

The catalytic washcoat coating may comprise a plurality of washcoatcoatings. For example, the catalyst washcoat coating may comprise afirst washcoat coating and a second washcoat coating. When there is aplurality of washcoat coatings, then at least one of the washcoatcoatings comprises at least one PGM.

By at least one “support material” herein, we mean a metal oxideselected from the group consisting of optionally stabilised alumina,amorphous silica-alumina, optionally stabilised zirconia, ceria,titania, an optionally stabilised ceria-zirconia mixed oxide and amolecular sieve or mixtures of any two or more thereof.

The at least one support material may include one or more molecularsieve, e.g. an aluminosilicate zeolite. The primary duty of themolecular sieve in the PGM catalyst for use in the present invention isfor improving hydrocarbon conversion over a duty cycle by storinghydrocarbon following cold start or during cold phases of a duty cycleand releasing stored hydrocarbon at higher temperatures when associatedplatinum group metal catalyst components are more active for HCconversion. See for example Applicant/Assignee's EP 0830201. Molecularsieves are typically used in catalyst compositions according to theinvention for light-duty diesel vehicles, whereas they are rarely usedin catalyst compositions for heavy duty diesel applications because theexhaust gas temperatures in heavy duty diesel engines mean thathydrocarbon trapping functionality is generally not required.

However, molecular sieves such as aluminosilicate zeolites are notparticularly good supports for platinum group metals because they aremainly silica, particularly relatively higher silica-to-aluminamolecular sieves, which are favoured for their increased thermaldurability: they may thermally degrade during ageing so that a structureof the molecular sieve may collapse and/or the PGM may sinter, givinglower dispersion and consequently lower HC and/or CO conversionactivity.

Accordingly, it is preferred that the catalytic washcoat coatingcomprises a molecular sieve at ≦30% by weight (such as ≦25% by weight,≦20% by weight e.g. ≦15% by weight) of the individual washcoat coatinglayer. When the catalytic washcoat coating comprises a first washcoatcoating and a second washcoat coating, then the first washcoat coatingand/or the second washcoating coating preferably coating comprise amolecular sieve at ≦30% by weight (such as ≦25% by weight, ≦20% byweight e.g. ≦15% by weight) of each individual washcoat coating layer.

Typically, the catalytic washcoat coating comprises a support material(e.g. a support material for the PGM). The support material may compriseor consist of a metal oxide selected from the group consisting ofoptionally stabilised alumina, amorphous silica-alumina, optionallystabilised zirconia, ceria, titania and an optionally stabilisedceria-zirconia mixed oxide or mixtures of any two or more thereof.Suitable stabilisers include one or more of silica and rare earthmetals.

When there is a first washcoat coating and a second washcoat coating,then the first washcoat coating and/or the second washcoat coating maycomprise at least one support material. Typically, the support materialcomprises a metal oxide selected from the group consisting of optionallystabilised alumina, amorphous silica-alumina, optionally stabilisedzirconia, ceria, titania and an optionally stabilised ceria-zirconiamixed oxide or mixtures of any two or more thereof. Suitable stabilisersinclude one or more of silica and rare earth metals.

Preferred molecular sieves for use as support materials/hydrocarbonadsorbers are medium pore zeolites, preferably aluminosilicate zeolites,i.e. those having a maximum ring size of eight tetrahedral atoms, andlarge pore zeolites (maximum of ten tetrahedral atoms) preferablyaluminosilicate zeolites, include natural or synthetic zeolites such asfaujasite, clinoptilolite, mordenite, silicalite, ferrierite, zeolite X,zeolite Y, ultrastable zeolite Y, ZSM-5 zeolite, ZSM-12 zeolite, SSZ-3zeolite, SAPO-5 zeolite, offretite or a beta zeolite, preferably ZSM-5,beta and Y zeolites. Preferred zeolite adsorbent materials have a highsilica to alumina ratio, for improved hydrothermal stability. Thezeolite may have a silica/alumina molar ratio of from at least about25/1, preferably at least about 50/1, with useful ranges of from about25/1 to 1000/1, 50/1 to 500/1 as well as about 25/1 to 100/1, 25/1 to300/1, from about 100/1 to 250/1.

Typically, the at least one material for trapping volatilised PGM of thethird substrate monolith comprises a metal oxide selected from the groupconsisting of optionally stabilised alumina, optionally stabilisedzirconia, an optionally stabilised ceria-zirconia mixed oxide andmixtures of any two or more thereof. Suitable stabilisers include one ormore of silica and rare earth metals. It is preferred that the metaloxide is selected from the group consisting of optionally stabilisedalumina and optionally stabilised ceria-zirconia mixed oxide. Theinventors have found that particularly alumina and ceria-containingmetal oxides per se are capable of trapping volatilised PGMs,particularly ceria, which has a particular affinity for Pt.

In one embodiment, the third substrate monolith does not comprise, orthe at least one material for trapping volatilised PGM is not, ceriumoxide or a perovskite material, such as CaTiO₃.

The at least one material for trapping volatilised PGM of the thirdsubstrate monolith may be a component of an extruded substrate monolith.However, it is preferred that the at least one material for trappingvolatilised PGM of the third substrate monolith is applied as acomponent of a washcoat coating to an inert substrate monolith.

Typically, the third substrate monolith comprises a total amount of thematerial for trapping volatilised PGM of 0.1 to 5 g in⁻³, preferably 0.2to 4 g in⁻³ (e.g. 0.5 to 3.5 g in⁻³), such as 1 to 2.5 g in⁻³.

In general, at least one second substrate monolith preferably comprisesplatinum (e.g. the at least one platinum group metal (PGM) of thecatalytic washcoat coating comprises platinum). When at least one PGM ina second substrate monolith is platinum, then the platinum is a PGMliable to volatilise when the second substrate monolith is exposed torelatively extreme conditions including relatively high temperatures.The relatively extreme conditions including relatively high temperaturesare, for example, temperatures of ≧700° C., preferably ≧800° C., or morepreferably ≧900° C.

Typically, at least one second substrate monolith comprises bothplatinum and palladium (e.g. the at least one platinum group metal (PGM)of the catalytic washcoat coating is both platinum and palladium). Theplatinum and/or the palladium can be the PGM liable to volatilise whenthe second substrate monolith is exposed to relatively extremeconditions including relatively high temperatures. However, when bothplatinum and palladium are present, then normally platinum is morelikely to be the PGM liable to volatilise when the first washcoatcoating is exposed to relatively extreme conditions including relativelyhigh temperatures.

In the second substrate monolith, it is possible for relatively highPt:Pd weight ratios to be used in the catalytic washcoat coating, suchas the first washcoat coating, for the purposes of, e.g. generating NO₂to promote downstream combustion of filtered particulate matter.

Preferably, the weight ratio of Pt:Pd is ≦10:1, e.g. 8:1, 6:1, 5:1 or4:1. Such relatively high weight ratios are permissible because anyvolatilised PGM is trapped on the third substrate monolith.

It is preferred that in the second substrate monolith the weight ratioof Pt:Pd is ≦2, such as ≦1.5:1, e.g. about 1:1. The inventors have foundthat it is possible to further reduce or prevent migration of PGM fromthe at least one second substrate monolith to a downstream SCR catalystby adopting such Pt:Pd weight ratios for the second substrate monolith.These ratios have been found to further reduce PGM volatilisation. Thesignificance of this feature is shown in the Examples: the inventorshave found that the preferred Pt:Pd weight ratios volatilise less, byempiric testing, than a similar catalyst having a Pt:Pd weight ratio of4:1. In layered catalyst arrangements (e.g. when the catalytic washcoatcoating comprises a plurality of washcoat coatings, such as a firstwashcoat coating and a second washcoat coating), it is preferred that anouter layer has a Pt:Pd weight ratio of ≦2, or optionally that theoverall Pt:Pd weight ratio of all layers combined is ≦2.

Typically, the weight ratio of Pt:Pd is ≧35:65 (e.g. ≧7:13). It ispreferred that the weight ratio Pt:Pd is ≧40:60 (e.g. ≧2:3), morepreferably ≧42.5:57.5 (e.g. ≧17:23), particularly ≧45:55 (e.g. ≧9:11),such as ≧50:50 (e.g. ≧1:1), and still more preferably ≧1.25:1. Theweight ratio of Pt:Pd is typically 10:1 to 7:13. It is preferred thatthe weight ratio of Pt:Pd is 8:1 to 2:3, more preferably 6:1 to 17:23,even more preferably 5:1 to 9:11, such as 4:1 to 1:1, and still morepreferably 2:1 to 1.25:1.

Generally, the total amount of the platinum group metal (PGM) (e.g. thetotal amount of Pt and/or Pd) is 1 to 500 g ft⁻³. Preferably, the totalamount of the PGM is 5 to 400 g ft⁻³, more preferably 10 to 300 g ft⁻³,still more preferably, 25 to 250 g ft⁻³, and even more preferably 35 to200 g ft⁻³.

The at least one material for trapping volatilised PGM of the thirdsubstrate monolith may comprise a catalyst composition comprising atleast one metal selected from the group consisting of palladium, copper,silver, gold and combinations of two or more thereof, supported on atleast one support material. The washcoat of the third substrate monolithcan comprise, with advantage, a washcoat coating comprising bothplatinum and palladium at a lower Pt:Pd weight ratio than the or eachwashcoat of the or each second substrate monolith, preferably at a Pt:Pdmolar ratio of ≦2, such as ≦1.5, e.g. about 1:1, or an alloy of Pd—Au.

The exhaust system according to the invention may further comprise aninjector for injecting a nitrogenous reductant (e.g. ammonia, or aprecursor thereof, such as urea) into a flowing exhaust gas between thefirst substrate monolith and the second substrate monolith.Alternatively, (i.e. without means for injecting ammonia or a precursorthereof such as urea is disposed between the first catalysed substratemonolith and the second catalysed substrate monolith), or in addition tothe means for injecting a nitrogenous reductant (e.g. ammonia or aprecursor thereof, such as urea), engine management means may beprovided for enriching exhaust gas, such that ammonia gas is generatedin situ by reduction of NO_(x) on the PGM catalyst of the or each secondsubstrate monolith.

Nitrogenous reductants and precursors thereof for use in the presentinvention include any of those mentioned hereinabove in connection withthe background section. Thus, for example, the nitrogenous reductant ispreferably ammonia or urea.

In combination with an appropriately designed and managed dieselcompression ignition engine, enriched exhaust gas, i.e. exhaust gascontaining increased quantities of carbon monoxide and hydrocarbonrelative to normal lean running mode, contacts the NAC. Componentswithin a NAC such as PGM-promoted ceria or ceria-zirconia can promotethe water-gas shift reaction, i.e. CO_((g))+H₂O_((v))→CO_(2(g))+H_(2(g))evolving H₂. From the side reaction footnote to reactions (3) and (4)set out hereinabove, e.g. Ba(NO₃)₂+8H₂→BaO+2NH₃+5 H₂O, NH₃ can begenerated in situ and stored for NO_(x) reduction on the downstream SCRcatalyst.

Typically, the means for injecting is arranged to inject nitrogenousreductant or a precursor thereof into a flowing exhaust gas between theor each second substrate monolith and the third substrate monolith. Inthis arrangement, the third substrate monolith may include a SCRcatalyst comprising copper. This has the advantages that the thirdsubstrate monolith can act as a SCR catalyst, a hydrolysis catalyst,i.e. to hydrolyse a nitrogenous reductant precursor, e.g. urea toammonia and water, and a PGM trap, and can also assist with mixing anddistribution of the nitrogenous reductant in a flowing exhaust gas.

Alternatively, the means for injecting can be arranged to injectnitrogenous reductant or a precursor thereof into a flowing exhaust gasbetween the third substrate monolith and the first substrate monolith.

The or each at least one second substrate monolith may be individuallyselected from an oxidation catalyst or a NO_(x) Absorber Catalyst (NAC).Generally, a NAC contains significant quantities of alkaline earthmetals and/or alkali metals relative to an oxidation catalyst. The NACtypically also includes ceria or a ceria-containing mixed oxide, e.g. amixed oxide of cerium and zirconium, which mixed oxide optionallyfurther including one or more additional lanthanide or rare earthelements. The oxidation catalyst generally has a composition asdescribed in the background set out hereinabove.

The or each first, second and/or third substrate monolith for use in theinvention can be a flow-through substrate monolith or a filteringsubstrate monolith having inlet surfaces and outlet surfaces, whereinthe inlet surfaces are separated from the outlet surfaces by a porousstructure. It is preferred that the filtering substrate monolith in eachcase is a wall-flow filter, i.e. a ceramic porous filter substratecomprising a plurality of inlet channels arranged in parallel with aplurality of outlet channels, wherein each inlet channel and each outletchannel is defined in part by a ceramic wall of porous structure,wherein each inlet channel is alternatingly separated from an outletchannel by a ceramic wall of porous structure and vice versa. In otherwords, the wall-flow filter is a honeycomb arrangement defining aplurality of first channels plugged at an upstream end and a pluralityof second channels not plugged at the upstream end but plugged at adownstream end. Channels vertically and laterally adjacent to a firstchannel are plugged at a downstream end. When viewed from either end,the alternately plugged and open ends of the channels take on theappearance of a chessboard.

Catalysed filters, preferably wall-flow filters, can be coated using themethod disclosed in Applicant/Assignee's WO 2011/080525. That is, amethod of coating a honeycomb monolith substrate comprising a pluralityof channels with a liquid comprising a catalyst component, which methodcomprising the steps of: (i) holding a honeycomb monolith substratesubstantially vertically; (ii) introducing a pre-determined volume ofthe liquid into the substrate via open ends of the channels at a lowerend of the substrate; (iii) sealingly retaining the introduced liquidwithin the substrate; (iv) inverting the substrate containing theretained liquid; and (v) applying a vacuum to open ends of the channelsof the substrate at the inverted, lower end of the substrate to draw theliquid along the channels of the substrate. The catalyst composition maybe coated on filter channels from a first end, following which thecoated filter can be dried.

Use of such a method can be controlled using e.g. vacuum strength,vacuum duration, washcoat viscosity, washcoat solids, coating particleor agglomerate size and surface tension so that catalyst is coatedpredominantly on the inlet surfaces but also optionally within theporous structure but near to the inlet surfaces. Alternatively, thewashcoat components may be milled to a size e.g. D90<5 μm, so that they“permeate” the porous structure of the filter (see WO 2005/016497).

The first substrate monolith comprises a catalyst for selectivelycatalysing the reduction of oxides of nitrogen to dinitrogen with anitrogenous reductant, also known as a selective catalytic reduction(SCR) catalyst.

The SCR catalyst may be coated as a coating onto a substrate monolith,such as described hereinabove. Alternatively, the SCR catalyst isprovided as an extrudate (also known as a “catalyst body”), i.e. thecatalyst is mixed with components of the substrate monolith structure,which are both extruded, so the catalyst is part of the walls of thesubstrate monolith.

The SCR catalyst of the first substrate monolith can comprise afiltering substrate monolith, preferably a wall-flow filter, or aflow-through substrate monolith. It is also possible to make a wall-flowfilter from an extruded SCR catalyst (see Applicant/Assignee's WO2009/093071 and WO 2011/092521).

SCR catalysts can be selected from the group consisting of at least oneof Cu, Hf, La, Au, In, V, lanthanides and Group VIII transition metals,such as Fe, supported on a refractory oxide or molecular sieve. Suitablerefractory oxides include Al₂O₃, TiO₂, CeO₂, SiO₂, ZrO₂ and mixed oxidescontaining two or more thereof. Non-zeolite catalyst can also includetungsten oxide, e.g. V₂O₅/WO₃/TiO₂. Preferred metals of particularinterest are selected from the group consisting of Ce, Fe and Cu.Molecular sieves can be ion-exchanged with the above metals.

It is preferred that the at least one molecular sieve, is analuminosilicate zeolite or a SAPO. The at least one molecular sieve canbe a small, a medium or a large pore molecular sieve, for example. By“small pore molecular sieve” herein we mean a molecular sievescontaining a maximum ring size of 8 tetrahedral atoms, such as CHA; by“medium pore molecular sieve” herein we mean a molecular sievecontaining a maximum ring size of 10 tetrahedral atoms, such as ZSM-5;and by “large pore molecular sieve” herein we mean a molecular sievehaving a maximum ring size of 12 tetrahedral atoms, such as beta. Smallpore molecular sieves are potentially advantageous for use in SCRcatalysts—see for example Applicant/Assignee's WO 2008/132452. Molecularsieves for use in SCR catalysts according to the invention include oneor more metals incorporated into a framework of the molecular sieve,e.g. Fe “in-framework” Beta and Cu “in-framework” CHA.

Particular molecular sieves with application in the present inventionare selected from the group consisting of AEI, ZSM-5, ZSM-20, ERIincluding ZSM-34, mordenite, ferrierite, BEA including Beta, Y, CHA, LEVincluding Nu-3, MCM-22 and EU-1, with CHA molecular sieves currentlypreferred in combination with Cu as a promoter, e.g. ion-exchanged.

The invention also provides a lean-burn internal combustion engine. Thelean-burn internal combustion engine can be a positive ignition, e.g. aspark ignition, engine that typically run on gasoline fuel or blends ofgasoline fuel and other components such as ethanol, but is preferably acompression ignition, e.g. a diesel-type engine. Lean-burn internalcombustion engines include homogenous charge compression ignition (HCCI)engines, powered either by gasoline etc. fuel or diesel fuel.

The engine may comprise engine management means arranged, when in use,to contact the filter with an enriched exhaust gas for generatingammonia in situ.

An exhaust system of the invention is shown in FIG. 6A. Exhaust system10 comprises, in serial arrangement from upstream to downstream, acatalysed wall-flow filter 4 coated with a washcoat comprising platinumsupported on an particulate alumina support material (corresponding tothe “at least one second substrate monolith comprising a catalyticwashcoat coating comprising at least one PGM feature of claim 1) havingthe purpose, among others, of promoting reactions (1) and (6) herein; aflow-through substrate monolith 2 coated with palladium supported onparticulate alumina as a guard bed; a source of ammonia 7 comprising aninjector for an ammonia precursor, urea; and a flow-through substratemonolith 6 coated with a Fe/Beta SCR catalyst. Each substrate monolith2, 4, 6 is disposed in a metal container or “can” including coneddiffusers and they are linked by a series of conduits 3 of smaller crosssectional area than a cross sectional area of any of substrate monoliths2, 4, 6. The coned diffusers act to spread the flow of exhaust gasentering a housing of a “canned” substrate monolith so that the exhaustgas as a whole is directed across substantially the whole front “face”of each substrate monolith. Exhaust gas exiting substrate monolith 8 isemitted to atmosphere at “tail pipe” 5.

FIG. 6B shows an alternative embodiment of an exhaust system 20according to the present invention comprising, in serial arrangementfrom upstream to downstream, a flow-through substrate monolith 8 coatedwith a two-layered diesel oxidation catalyst composition comprising bothplatinum and palladium at an overall Pt:Pd ratio of 4:1 for the purposeof promoting reactions (1) and (6) herein; a source of ammonia 7comprising an injector for an ammonia precursor, urea; a flow-throughsubstrate monolith 2 coated with a ceria-zirconia mixed oxide in aceria:zirconia weight ratio of 9:1 as a guard bed; and a wall-flowfilter substrate monolith 9 coated with a Cu/CHA SCR catalyst. Eachsubstrate monolith 2, 8, 9 is disposed in a metal container or “can”including coned diffusers and they are linked by a series of conduits 3of smaller cross sectional area than a cross sectional area of any ofsubstrate monoliths 2, 8, 9. Exhaust gas exiting substrate monolith 8 isemitted to atmosphere at “tail pipe” 5. An advantage of the arrangementof the urea injector to between the second and third substrate monolithis that the third substrate monolith can act as a hydrolysis catalyst,i.e. to hydrolyse a nitrogenous reductant precursor, e.g. urea toammonia and water, and a PGM trap, and the third substrate monolith canalso assist with mixing and distribution of the nitrogenous reductant ina flowing exhaust gas upstream of the SCR catalyst.

EXAMPLES Example 1 Preparation of Substrate Monolith Coated with 5 wt %Fe/Beta Zeolite

Commercially available Beta zeolite was added to an aqueous solution ofFe(NO₃)₃ with stiffing. After mixing, binders and rheology modifierswere added to form a washcoat composition.

A 400 cells per square inch cordierite flow-through substrate monolithwas coated with an aqueous slurry of the 5 wt % Fe/Beta zeolite sampleusing the method disclosed in Applicant/Assignee's WO 99/47260, i.e.comprising the steps of (a) locating a containment means on top of asupport, (b) dosing a pre-determined quantity of a liquid component intosaid containment means, either in the order (a) then (b) or (b) then(a), and (c) by applying pressure or vacuum, drawing said liquidcomponent into at least a portion of the support, and retainingsubstantially all of said quantity within the support. This coatedproduct (coated from one end only) is dried and then calcined and thisprocess is repeated from the other end so that substantially the entiresubstrate monolith is coated, with a minor overlap in the axialdirection at the join between the two coatings. A core of 1 inch (2.54cm) diameter×3 inches long was cut from the finished article.

Comparative Example 2 Preparation of Pt-Only Catalysed Wall-Flow Filter

A washcoat composition comprising a mixture of alumina particles milledto a relatively high particle size distribution, platinum nitrate,binders and rheology modifiers in deionised water was prepared. Analuminium titanate wall-flow filter was coated with the catalystcomposition at a washcoat loading of 0.2 g/in³ to a final total Ptloading of 5 g/ft⁻³ using the method and apparatus disclosed in theApplicant/Assignee's WO 2011/080525, wherein channels at a first endintended for orientation to an upstream side were coated for 75% oftheir total length with a washcoat comprising platinum nitrate andparticulate alumina from the intended upstream end thereof; and channelsat an opposite end and intended to be oriented to a downstream side arecoated for 25% of their total length with the same washcoat as the inletchannels. That is, the method comprised the steps of: (i) holding ahoneycomb monolith substrate substantially vertically; (ii) introducinga pre-determined volume of the liquid into the substrate via open endsof the channels at a lower end of the substrate; (iii) sealinglyretaining the introduced liquid within the substrate; (iv) inverting thesubstrate containing the retained liquid; and (v) applying a vacuum toopen ends of the channels of the substrate at the inverted, lower end ofthe substrate to draw the liquid along the channels of the substrate.The catalyst composition was coated on filter channels from a first end,following which the coated filter was dried. The dried filter coatedfrom the first end was then turned and the method was repeated to coatthe same catalyst to filter channels from the second end, followed bydrying and calcining.

A core of 1 inch (2.54 cm) diameter×3 inches (7.62 cm) long was cut fromthe finished article. The resulting part is described as “fresh”, i.e.unaged.

Example 3A and 3B Preparation of Alumina Guard Beds

A 400 cells per square inch cordierite flow-through substrate monolithwas coated with an aqueous slurry comprising particulate alumina usingthe method disclosed in Applicant/Assignee's WO 99/47260. The solidscontent was selected to prepare two different washcoat loadings: a first(designated Example 3A) of 1.0 g/in³; and a second (3B) of 0.4 g/in³.The resulting parts were dried, then calcined.

A core of 1 inch (2.54 cm) diameter×3 inches (7.62 cm) long was cut fromthe finished article.

Example 4A and 4B Preparation of Ceria:Zirconia Mixed Oxide Guard Beds

A 400 cells per square inch cordierite flow-through substrate monolithwas coated with an aqueous slurry comprising a particulateceria:zirconia mixed oxide using the method disclosed inApplicant/Assignee's WO 99/47260. Two different ceria:zirconia mixedoxide materials were used. A first (designated Example 4A) had aceria:zirconia weight ratio of 9:1; whereas a second (4B) hadceria:zirconia weight ratio of 1:9. Both ceria:zirconia mixed oxideswere coated on the flow-through substrate monolith at a washcoat loadingof 1.0 g/in³. The resulting parts were dried, then calcined.

A core of 1 inch (2.54 cm) diameter×3 inches (7.62 cm) long was cut fromthe finished article.

Example 5A and 5B Preparation of Pd/Ceria:Zirconia Mixed Oxide GuardBeds

Samples prepared according to Examples 4A and 4B prior to core cuttingwere impregnated with an aqueous solution of palladium nitrate(designated as Examples 5A and 5B respectively). The resulting partswere then dried and calcined and a core of 1 inch (2.54 cm) diameter×3inches (7.62 cm) long was cut from each finished article.

The process was conducted in such a way as to obtain a palladium loadingin the final product of 5.0 g/ft³.

Example 6 Preparation of Pd/Alumina Guard Bed

Sample 3A prior to core cutting was impregnated with an aqueous solutionof palladium nitrate. The resulting part was then dried and calcined anda core of 1 inch (2.54 cm) diameter×3 inches (7.62 cm) long was cut fromeach finished article.

The process was conducted in such a way as to obtain a palladium loadingin the final product of 5.0 g/ft³.

Example 7 Preparation of 1:1 Weight % Pt:Pd Containing CatalysedWall-Flow Filter

A coated filter was prepared using the same method as in ComparativeExample 2, except in that the washcoat applied to both the inletchannels and the outlet channels of the filter included palladiumnitrate in addition to the platinum nitrate. The washcoat loading in theinlet and outlet channels was conducted in such a way as to arrive at a5 g/ft³ Pt, 5 g/ft³ Pd loading on both the inlet surfaces and the outletsurfaces, i.e. a total PGM loading of 10 g/ft³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 8 Preparation of 5:1 Weight % Pt:Pd Containing CatalysedWall-Flow Filter

A coated filter was prepared using the same method as in ComparativeExample 2, except in that the washcoat applied to both the inletchannels and the outlet channels of the filter included palladiumnitrate in addition to the platinum nitrate. The washcoat loading in theinlet and outlet channels was conducted in such a way as to arrive at a5 g/ft³ Pt, 1 g/ft³ Pd loading on both the inlet surfaces and the outletsurfaces, i.e. a total PGM loading of 6 g/ft³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 9 System Tests

The tests were performed on a first synthetic catalyst activity test(SCAT) laboratory reactor illustrated in FIG. 1, in which a fresh coreof the coated Fe/Beta zeolite SCR catalyst of Example 1 is disposed in aconduit downstream of a core of either the catalysed wall-flow filter ofComparative Example 2 (control) or of fresh cores of Comparative Example2 followed in turn by Guard Bed cores of Examples 3A, 3B, 4A, 4B, 5A, 5Bor 6. A synthetic gas mixture was passed through the conduit at acatalyst swept volume of 30,000 hr⁻¹. A furnace was used to heat (or“age”) the catalysed wall-flow filter sample at a steady-statetemperature at a filter inlet temperature of 900° C. for 60 minutes,during which the inlet SCR catalyst temperature was held at 300° C. TheGuard Bed cores were located in the furnace immediately downstream ofthe Comparative Example 2 core and heated to 900° C. (see also FIG. 1).An air (heat exchanger) or water cooling mechanism was used to effectthe temperature drop between the filter and the SCR catalyst. The gasmixture during the ageing was 10% O₂, 6% H₂O, 6% CO₂, 100 ppm CO, 400ppm NO, 100 ppm HC as C1, balance N₂.

Following ageing, the aged SCR catalysts were removed from the firstSCAT reactor and inserted into a second SCAT reactor specifically totest NH₃—SCR activity of the aged samples. The aged SCR catalysts werethen tested for SCR activity at 150, 200, 250, 300, 350, 450, 550 and650° C. using a synthetic gas mixture (O₂=14%; H₂O=7%; CO₂=5%; NH₃=250ppm; NO=250 ppm; NO₂=0 ppm; N₂=balance) and the resulting NO_(x)conversion was plotted against temperature for each temperature datapoint in FIGS. 2 and 3. These plots essentially measure competitionbetween reaction (9) and reaction (5) and thus how much reaction (9)affects the NO_(x) conversion by consumption of the available NH₃ neededfor the SCR reaction (reaction (5)).

The results are shown in FIGS. 2 and 3. It can be seen from the resultsin FIG. 2 that the SCR catalyst behind the guard bed comprising the 9:1ceria:zirconia mixed oxide of Example 4A retained substantially all SCRactivity, whereas the SCR catalyst aged downstream of the guard bedcomprising 1:9 ceria:zirconia mixed oxide of Example 4B retained lessNO_(x) conversion activity. The results of the control experimentfeaturing Comparative Example 2 only (no guard bed) show that in theabsence of a guard bed, the NO_(x) conversion activity of the SCRcatalyst is substantially reduced. The inventors concluded that thisresult is explained by platinum from the catalysed soot filter ofComparative Example 2 volatilising and migrating to the downstream SCRcatalyst. The migrated platinum reduces net NO_(x) conversion by causingthe undesirable combustion of ammonia according to reaction (9). Hence,Example 4B is a poorer guard bed than Example 4A, as the SCR catalyst“guarded” by the guard bed of Example 4B retains less NO_(x) conversionactivity. Furthermore, the inventors concluded that, following the trendof the results for Examples 4B and 4A, 100% cerium oxide, i.e. ceria,would also perform similarly to the Example 4A sample. However, thechoice of whether to use ceria as such may be influenced by otherfactors, such as the desired thermal durability and/or tolerance tosulphur poisoning. Substantially no loss in activity was seen between afresh Fe/Beta SCR catalyst and a Fe/Beta SCR catalyst aged at 300° C.for 1 hour without any catalyst present upstream (results not shown).

Example 5B shows that the ability of Example 4B to protect the SCRcatalyst from being contacted by volatilised platinum is improved withthe addition of Pd. However, since the ability of Example 4A (nopalladium) to guard the SCR catalyst from volatilised platinum isalready significant, no additional improvement is seen when palladium isadded to the 9:1 ceria-zirconia of Example 4A. In summary, therefore,Example 4A shows that it is possible to guard a downstream SCR catalystfrom potential contact by volatilised platinum without recourse toexpensive additional components such as palladium.

Referring to FIG. 3, it can be seen from these results that alumina onlyhas some ability to protect a downstream SCR catalyst from contact byvolatilised platinum (Example 3B) and that such ability can be improvedby increasing the alumina washcoat loading and most preferably bysupporting palladium on the alumina. However, on a cost/benefitanalysis, the best result obtained in the Examples is to use the 9:1weight ratio of ceria-zirconia mixed oxide of Example 4A.

Example 10 Preparation of Substrate Monolith Coated with 3 Wt % Cu/CHAZeolite

Commercially available aluminosilicate CHA zeolite was added to anaqueous solution of Cu(NO₃)₂ with stirring. The slurry was filtered,then washed and dried. The procedure can be repeated to achieve adesired metal loading. The final product was calcined. After mixing,binders and rheology modifiers were added to form a washcoatcomposition.

A 400 cpsi cordierite flow-through substrate monolith was coated with anaqueous slurry of the 3 wt % Cu/CHA zeolite sample using the methoddisclosed in Applicant/Assignee's WO 99/47260 described in Example 1hereinabove. The coated substrate monolith was aged in a furnace in airat 500° C. for 5 hours. A core of 1 inch (2.54 cm) diameter×3 incheslong (7.62 cm) was cut from the finished article.

Example 11 Further Pt:Pd Weight Ratio Studies

Two diesel oxidation catalysts were prepared as follows:

Diesel Oxidation Catalyst A

A single layered DOC was prepared as follows. Platinum nitrate andpalladium nitrate were added to a slurry of silica-alumina. Beta zeolitewas added to the slurry such that it comprised <30% of the solidscontent as zeolite by mass. The washcoat slurry was dosed onto a 400cpsi flow-through substrate using the method of Example 10. The dosedpart was dried and then calcined at 500° C. The total platinum groupmetal loading in the washcoat coating was 60 gft⁻³ and the total Pt:Pdweight ratio was 4:1. A core of 1 inch (2.54 cm) diameter×3 inches (7.62cm) long was cut from the finished article. The resulting part may bedescribed as “fresh”, i.e. unaged.

Diesel Oxidation Catalyst B

A single layered DOC was prepared as follows. Platinum nitrate andpalladium nitrate were added to a slurry of silica-alumina. Beta zeolitewas added to the slurry such that it comprised <30% of the solidscontent as zeolite by mass. The washcoat slurry was dosed onto a 400cpsi flow-through substrate using the same method as used for DOC A. Thedosed part was dried and then calcined at 500° C. The total PGM loadingin the single layer DOC was 120 g/ft³ and the Pt:Pd weight ratio was2:1. A core of 1 inch (2.54 cm) diameter×3 inches (7.62 cm) long was cutfrom the finished article. The resulting part may be described as“fresh”, i.e. unaged.

Both catalysts were tested according the protocols set out in Example12. The results are set out in FIG. 5 with reference to a control (agedSCR catalyst that has not been further aged downstream of either DOC Aor DOC B).

Example 12 System Tests

The tests were performed on a first synthetic catalyst activity test(SCAT) laboratory reactor illustrated in FIG. 1, in which an aged coreof the coated Cu/CHA zeolite SCR catalyst of Example 1 was disposed in aconduit downstream of a core of either the Diesel Oxidation Catalyst(DOC) A or B (according to Example 11). A synthetic gas mixture waspassed through the conduit at a rate of 6 liters per minute. A furnacewas used to heat (or “age”) the DOC samples at a steady-statetemperature at a catalyst outlet temperature of 900° C. for 2 hours. TheSCR catalyst was disposed downstream of the DOC sample and was held at acatalyst temperature of 300° C. during the ageing process by adjustingthe length of tube between the furnace outlet and the SCR inlet,although a water cooled heat exchanger jacket could also be used asappropriate. Temperatures were determined using appropriately positionedthermocouples (T₁ and T₂). The gas mixture used during the ageing was40% air, 50% N₂, 10% H₂O.

Following the DOC ageing, the SCR catalysts were removed from the firstSCAT reactor and inserted into a second SCAT reactor specifically totest NH₃—SCR activity of the aged samples. The SCR catalysts were thentested for SCR activity at 500° C. using a synthetic gas mixture(O₂=10%; H₂O=5%; CO₂=7.5%; CO=330 ppm; NH₃=400 ppm; NO=500 ppm; NO₂=0ppm; N₂=balance, i.e. an alpha value of 0.8 was used (ratio ofNH₃:NO_(x)), so that the maximum possible NO_(x) conversion availablewas 80%) and the resulting NO_(x) conversion was plotted againsttemperature on the accompanying bar chart in FIG. 5. This plotessentially measures competition between reaction (9) and reaction (5)and thus how much reaction (9) affects the NO_(x) conversion byconsumption of the available NH₃ needed for the SCR reaction (reaction(5)).

Pt:Pd Weight Ratio Study—Conclusions

Taken as a whole, the results of Example 9 shown in FIG. 4 in connectionwith Examples 7 and 8 and Comparative Example 2 indicate that a Pt:Pdweight ratio of between 1:1 and 5:1 is beneficial in reducing theproblem of NO_(x) conversion activity loss through volatilisation ofplatinum group metal, principally platinum, from a platinum group metalcontaining catalyst to a downstream SCR catalyst; and

The results of Example 12 shown in FIG. 5 in connection with DieselOxidation Catalysts A and B show that for a SCR catalyst aged downstreamof a DOC having a 2:1 Pt:Pd weight ratio overall, the loss of NO_(x)conversion activity is relatively slight at 67% NO_(x) conversionactivity compared with the control at 72% NO_(x) conversion activity (aSCR catalyst aged behind a 1:1 Pt:Pd weight ratio overall DOC (notdescribed herein) using the same protocol had a NO_(x) conversionactivity of 69%). However, when the overall Pt:Pd weight ratio wasincreased to 4:1, SCR activity was significantly reduced to 48%.

The inventors conclude, therefore, that there exists a boundary at about2:1 Pt:Pd weight ratio overall above which Pt volatilisation is morelikely to occur. Hence, by limiting to an overall Pt:Pd weight ratio of2:1 in the DOC as a whole, and to ≦2:1 Pt:Pd weight ratio in the secondwashcoat coating layer, Pt in the DOC is less likely to volatilise andmigrate to a downstream SCR catalyst.

For the avoidance of any doubt, the entire contents of any and alldocuments cited herein are incorporated herein by reference in theirentirety.

The invention claimed is:
 1. An exhaust system for a vehicular lean-burninternal combustion engine, which system comprising: (a) a firstsubstrate monolith comprising a SCR catalyst; (b) at least one secondsubstrate monolith comprising a catalytic washcoat coating comprising atleast one platinum group metal (PGM), which at least one secondsubstrate monolith is disposed upstream of the first substrate monolith;and (c) a third substrate monolith disposed between the first substratemonolith and the or each second substrate monolith, wherein at least onePGM on the or each second substrate monolith is liable to volatilisewhen the or each second substrate monolith is exposed to relativelyextreme conditions including relatively high temperatures, and whereinthe third substrate monolith comprises at least one material fortrapping volatilised PGM, and wherein: (i) the first substrate monolithis a filtering substrate monolith having inlet surfaces and outletsurfaces, wherein the inlet surfaces are separated from the outletsurfaces by a porous structure; or (ii) the second substrate monolith oreach second substrate monolith is a filtering substrate monolith havinginlet surfaces and outlet surfaces, wherein the inlet surfaces areseparated from the outlet surfaces by a porous structure.
 2. The exhaustsystem according to claim 1, wherein the at least one material fortrapping volatilised PGM comprises a metal oxide selected from the groupconsisting of optionally stabilised alumina, optionally stabilisedzirconia, an optionally stabilised cerin-zirconia mixed oxide andmixtures of any two or more thereof.
 3. The exhaust system according toclaim 2, wherein the at least one material for trapping volatilised PGMis a component of a washcoat coating applied to the third substratemonolith.
 4. The exhaust system according to claim 1, wherein at leastone second substrate monolith comprises platinum.
 5. The exhaust systemaccording to claim 1, wherein the at least one material for trappingvolatilised PGM comprises a catalyst composition comprising at least onemetal selected from the group consisting of palladium, copper, silver,gold and combinations of any two or more thereof, supported on at leastone support material.
 6. The exhaust system according to claim 5,wherein the third substrate monolith comprises a washcoat coatingcomprising both platinum and palladium at a lower Pt:Pd weight ratiothan the or each washcoat of the or each second substrate monolith. 7.The exhaust system according to claim 6, wherein a weight ratio of Pt:Pdin the washcoat coating of the third substrate is ≦2.
 8. An exhaustsystem for a vehicular lean-burn internal combustion engine, whichsystem comprising: (a) a first substrate monolith comprising a SCRcatalyst; (b) at least one second substrate monolith comprising acatalytic washcoat coating comprising at least one platinum group metal(PGM), which at least one second substrate monolith is disposed upstreamof the first substrate monolith; and (c) a third substrate monolithcomprising a washcoat coating comprising both platinum and palladium ata lower Pt:Pd weight ratio than the or each washcoat of the or eachsecond substrate monolith, wherein the third substrate monolith isdisposed between the first substrate monolith and the or each secondsubstrate monolith, wherein at least one PGM on the or each secondsubstrate monolith is liable to volatilise when the or each secondsubstrate monolith is exposed to relatively extreme conditions includingrelatively high temperatures, and wherein the third substrate monolithcomprises at least one material for trapping volatilised PGM, which atleast one material comprises a catalyst composition comprising at leastone metal selected from the group consisting of palladium, copper,silver, gold and combinations of any two or more thereof, supported onat least one support material.
 9. The exhaust system according to claim8, wherein at least one second substrate monolith comprises platinum.10. An exhaust system for a vehicular lean-burn internal combustionengine, which system comprising: (a) a first substrate monolithcomprising a SCR catalyst; (b) at least one second substrate monolithcomprising a catalytic washcoat coating comprising at least one platinumgroup metal (PGM), wherein the at least one platinum group metal (PGM)comprises both platinum and palladium and a weight ratio of Pt:Pd is ≦2,and wherein at least one second substrate monolith is disposed upstreamof the first substrate monolith; and (c) a third substrate monolithdisposed between the first substrate monolith and the or each secondsubstrate monolith, wherein at least one PGM on the or each secondsubstrate monolith is liable to volatilise when the or each secondsubstrate monolith is exposed to relatively extreme conditions includingrelatively high temperatures, and wherein the third substrate monolithcomprises at least one material for trapping volatilised PGM.
 11. Theexhaust system according to claim 10, wherein the at least one materialfor trapping volatilised PGM comprises a metal oxide selected from thegroup consisting of optionally stabilised alumina, optionally stabilisedzirconia, an optionally stabilised cerin-zirconia mixed oxide andmixtures of any two or more thereof.
 12. The exhaust system according toclaim 11, wherein the at least one material for trapping volatilised PGMis a component of a washcoat coating applied to the third substratemonolith.